pH effects on cardiac function and systemic vascular resistance in preterm infants

Abstract

Objective: To investigate the effect of pH on cardiac function and systemic vascular resistance (SVR) in preterm infants.

Study design: In this prospective observational study, we evaluated hemodynamically stable, ≤ 30 weeks' gestation preterm infants during the first 2 postnatal weeks. Cardiac function was assessed by echocardiography at the time of arterial blood draw for clinically indicated blood gas analysis. Data were separately analyzed for the transitional (days 1-3) and post-transitional (days 4-14) periods.

Results: We evaluated 147 pairs of arterial blood gases and echocardiograms in 29 preterm neonates (gestational age = 26.2 ± 1.5 weeks). Arterial pH ranged from 7.02-7.46. There was no linear relationship between pH and shortening fraction or stress-velocity index in transitional or post-transitional periods. We found a weak negative linear relationship between pH and left ventricular output and a positive linear relationship between pH and SVR only during the post-transitional period. These relationships were maintained after adjustment for the degree of base deficit. Arterial CO2 had effects similar to pH on myocardial function.

Conclusions: Unlike adults, myocardial contractility remains relatively unaffected by acidosis even at pH values close to 7.00 in hemodynamically stable preterm neonates during the first 2 postnatal weeks. However, as in adults, worsening acidosis in preterm neonates after the immediate transitional period is associated with a decrease in SVR along with an increase in left ventricular output. Thus, although myocardial contractility remains unaffected in preterm neonates during the first 2 postnatal weeks, the vascular response to acidosis undergoes a relatively rapid postnatal maturational process.